Best: Wireguard VPN, very easy to setup using PiVPN

Second best: DDNS with port forwarding and a strong password

Optional: Install XRDP for GUI and connect using Remote Desktop instead of SSH

As someone who does something similar the thing is, bulk will kill your margins and otherwise you’ll have to watch Kleinanzeigen like a hawk for 12 hours a day if you want to keep it private to private. If you increase volume you will also increase overhead and costs, especially if you reach a size where you’ll have to open up a Gewerbe and give Gewährleistung.

My advice to you would be

• keep your volume low and try to only do high value trades. Below 50€ guaranteed profit I won’t even leave the house anymore, below 100€ I will only drive out if I have nothing else to do that’s more important at the moment.
• Try to expand into general electronics and just watch Kleinanzeigen as often as you have the time, and only buy stuff that will sell for at least 50-100€ more guaranteed (so basically lowest price in all of Germany minus 50-100€ will ave to be your purchase price) Thisgives you pricing flexibility
• Try to convince people to drive to you, for most people receiving 20 or 50€ more than they asked sounds super great, for you it saves 1-2 hours and fuel costs that might be similar to or higher than what you pay the seller. Only ask or offer this after you’ve already negotiated the lowest pickup price though
• Best time to buy is the beginning of the month, best time to sell is end of the month, don’t keep so much inventory that you go broke during the month

And so on and so on, just a small slice of some “pro tips”, let me know if you want more tips, talk business or join powers and work together